my feelings are the difference between 2 and 3 cas latency is in nanoseconds.....you aren't ever gonna notice that....only a computer can register and acknowledge nanoseconds
EDIT: The 8 on the end is probably a restriction factor, BUT there is an exception to that rule...when people overclock..they have to loosen their ram timings so that it'll run stable. Some people will have an insanely high overclocked computer and you look at their ram timings and it'll be like 3-4-4-11, I think 11 is the highest I've ever seen it at though.